In recent years, materials for forming color images in particular have been mainstream image-recording materials, and more specifically, inkjet system of recording materials, thermal transfer system of recording materials, electro photographic recording materials, transfer system of silver halide photosensitive materials, printing ink, recording pens and the like have been active in use.
Colorants used in ink compositions are required to have good solubility or dispersability in solvents, to ensure high-density recording, to have good hues, to provide printed images of good storage stability, to excel in fastness to water and chemical agents, to have no toxicity, to be high in purity, and further to be available at low cost.
Recent technological advances have allowed improvements of monochromatic image's fastness to light, heat and active gases in surroundings (e.g. SOx in addition to oxidizing gases such as NOx and ozone) through the enhancement of electric potential of colorant molecules (JP-A-2007-63520). On the other hand, performance requirements for ink compositions include e.g. not only prevention of colorant precipitation from ink compositions, freeze of ink compositions (JP-A-2001-271013), curling at the time of printing of ink compositions on substrates (JP-A-6-240189) and decomposition of ink compositions (JP-A-6-234943), but also protection of colorants in the ink compositions from smearing at the time of printing (JP-A-6-136309), and these challenges have been solved by use of various additives in ink compositions.
Tries to improve properties of colorants and storage stability of ink compositions through the use of additives have been made as described in JP-A-2007-70566, and improvements in stabilization of dissolved colorants have been obtained.
On the other hand, techniques to use a variety of color materials in combination are described in JP-A-2007-191644, JP-A-2007-302810 and JP-A-2008-31261, but these techniques cannot satisfy even the long-term storage stability of ink compositions.